Author Topic: DIY Measurement fuel quantity and EGT  (Read 1641 times)

Offline AlexS

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DIY Measurement fuel quantity and EGT
« on: October 23, 2018, 10:14:40 PM »
My car can drive both on petrol and on LPG. Currently I could not measure the amount of LPG. I tank after a fixed number of kilometers. :thinking:

It seemed like a fun and useful project to make this measurable. There are various measurement systems for sale. But after my knowledge these are not as accurate and quite pricey.

Given this reason, I have been busy with a measurement system on an arduino board with LCD display.


The LPG tank is mounted on a matching frame in the trunk of the car. The idea is that the weight of the tank and frame is measured. This is realized by two load cells.
These load cells are mounted on the frame of the LPG tank.
The total weight of the tank is not fully absorbed by the load cells.
It is important to calibrate the whole.

When the tank is empty, the whole can be calibrated as a 'start value' to show 0 liters of liquid. This is also called the offset value. The second is calibrating with a quantity of fuel, as being 'scale value'.

As shown in the images, the amount is still displayed in kilograms on the LCD screen. This needs to be adjusted with the accompanying density of the liquid.

Given that there are still entries of the arduino available, I also added a thermocouple to read the exhaust gas temperature of the engine. This seemed nice to me to read.


The script and system are running. Now install it in the car.

If someone is interested in the script and wiring, you are free to ask!

Offline Admiral_dk

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Re: DIY Measurement fuel quantity and EGT
« Reply #1 on: October 24, 2018, 09:50:55 PM »
Interesting project .... BUT - I really hope that you made very sure that the LPG tank can't move or being converted into at projectile in case of an accident ....

I'm pretty sure you would get into a lot of trouble here in Denmark if you use the vehicle on public roads with your modifications ...!

Best wishes

Per

Offline crueby

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Re: DIY Measurement fuel quantity and EGT
« Reply #2 on: October 24, 2018, 10:19:19 PM »
There are more and more vehicles (mostly commercial ones and cityy busses) here in the US that run on LPG (and natural gas I think). Do you know how they measure the fuel left in those? Also by weight, or is there a way to do it with pressure?

Offline Admiral_dk

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Re: DIY Measurement fuel quantity and EGT
« Reply #3 on: October 25, 2018, 11:15:18 AM »
Our man in Thermo Dynamics might know about this subject.

I do know that you can get very accurate flow sensors for this purpose - but they cost a minor fortune ….

Offline MJM460

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Re: DIY Measurement fuel quantity and EGT
« Reply #4 on: October 25, 2018, 12:39:39 PM »
OK, that looks like my cue!

LPG is Liquified Petroleum Gas is essentially propane (with minor amounts of butane and ethane) which gives the fuel enough density to allow carrying sufficient for a reasonable range, while still at moderate pressure.  The liquid boils to vapour as it is drawn off to fuel the engine.  The term LPG actually applies to mixtures of propane and butane in any proportion, but most automotive fuel is mostly propane.

As a boiling liquid, the pressure is determined only by the temperature, and essentially, while any liquid remains in the tank, the pressure remains nearly constant.

I say nearly, because it takes energy to evaporate the fuel drawn off, and in the absence of any heat source, the remaining liquid has to supply the heat, which makes it cooler.  This lowering of temperature lowers the pressure, but the pressure change is a measure of the rate fuel is being drawn off, and of course increases as the quantity of heat is drawn from an ever decreasing mass remaining.  But still not a measure of the remaining mass.

The busses that run on natural gas (essentially methane) are different, in that I don't believe you can liquefy methane unless the temperature is below about -80 C, and then the pressure would be quite high.  Conventional LNG plants which keep LNG in large tanks operate at atmospheric pressure and a temperature around - 160 C, not very practical in a car.  I believe the refuelling stations have very high pressure compressors, so I think it is just stored in a bank of very high pressure cylinders.  Similarly for hydrogen powered vehicles.

However there are technologies involving absorption of the fuel in some sort of sponge like material where there is some sort of physical bonding which allows storage at more moderate pressures, similar to acetylene cylinders.  This involves different physics from simple boiling of a liquid, or gas laws for an ideal gas under pressure, and I don't know any more about those systems.

The fancy gadgets that you can buy to measure the liquid remaining in your bar-b-que gas bottle mostly rely on detecting the temperature change between liquid and vapour so only work when the fire is burning.  Some look like pressure gauges, and I don't know how, or even if they work over the full range of the bottle, or perhaps they just show the fall in pressure when the liquid runs out.  Not much use, as the bottle is then essentially empty.

So I believe the solutions lie in either weighing systems, with the attendant problems of allowing for vehicle motion and retention of the bottle in an accident as already mentioned, or a volume meter.  Turbine meters are used for larger quantities, and I assume some sort of positive displacement meter for typical automotive fuel consumption.  I can believe these would both be quite expensive unless they were really mass produced for the auto industry.  And I assume if you want to buy even a liquid fuel meter for your car as behind the typical fuel computer in modern cars, it would be a quite expensive meter, but still not suitable for LPG vapour and pressure.

MJM460
The more I learn, the more I find that I still have to learn!

Offline AlexS

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Re: DIY Measurement fuel quantity and EGT
« Reply #5 on: October 25, 2018, 10:52:41 PM »
Thank you guys. You have pointed out an important point! This is also the most difficult issue to properly fix the tank. And it can indeed cause problems during an accident. The tank is mounted an older car in the trunk.

The idea is to mount the frame and tank straps on the original frame. Bolted on the flour of the trunk, but while maintaining some up and down movement for measurement. So technical, the tank can't move any way. Only a bit up and down (also not much space between tank an topside trunk). The setup shown in the first picture. Could place the sensor on two places , 1 and two shown in the pic.
But at the end, it is a own modification and not legal. Have to check this.

MJM460, thanks for your extensive explanation. I have heard that LPG typically contains 60% propane and 40% methane and filled with 8 bar pressure? After the temperature decreases, more propane is mixed. Kind of mass flow sensors are for this typical rate of gases way to expensive and complicated to use. The ones I know are for industrial and scientific purposes.
The LPG in my car is sucked in as gaseous with the help of evaporation.In modern cars LPG is mainly injected as liquid. The consumption of the fuel can thus be better determined on the basis of injection pressure, opening and injection time. But it would be nice that I can measure the quantity of the fuel, and if possible more accurate.

The measurement systems I know are kind of the type shown in the last two pictures. If I am right (someone told me), it seems like that the the magnet in the tank act like a 'float'. And a sensor measures the strength of the magnetic field. These senors are likely to fail and not linear and fluctuating values seen rocking the tank.
I could fix this sensor, but I am open for maybe other idea ;)

Offline Admiral_dk

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Re: DIY Measurement fuel quantity and EGT
« Reply #6 on: October 26, 2018, 12:30:27 PM »
Texas Instruments (TI) makes an interesting line of IC's DRV4xx that can measure magnetic flux very accurate - but I will admit that I haven't got a clue if any of them are what you could be looking for, as distance measurement to float ….

Offline MJM460

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Re: DIY Measurement fuel quantity and EGT
« Reply #7 on: October 26, 2018, 12:36:49 PM »
Hi Alex, I would suspect that that fuel mixture might be propane - butane with only trace quantities of methane and slight larger trace quantities of ethane based on the vapour pressures. 

Eight bar (gauge or absolute?) corresponds to the vapour pressure of pure propane at about 20 to 25 deg C, so would not be sufficient to fill a fuel tank on a warm day.  However, the addition of 40% butane would lower the vapour pressure of the mixture.  In winter, I assume you have snow, and the temperature would not be sufficient to evaporate the fuel in the tank, though direct injection of liquid would overcome that issue.  But they might also use more propane and less butane in winter. Liquid injection would also overcome the issue of condensation on the outside of the tank due to the cooling which occurs to evaporate the fuel as it is being used, though I believe the normal scheme is to draw liquid from the tank, then evaporate it closer to the engine where there is more heat available.  Also overcomes the condensation issue.

I am unable to tell you the exact properties of a propane - methane mix, though it is easy enough for a chemical engineer, but pure methane vapour pressure is so high that it is not possible to get liquid above about -80 C.  While mixing with propane would raise that temperature a little, I really can't see getting 40% methane in a propane mix at any reasonable pressure at normal ambient temperature.

For comparison, our LPG gas bottles are pressure tested to 3300 kPa.  Based on 1.5 times operating pressure, this corresponds to about 2200 kPa maximum operating pressure, or 22 bar, which is the vapour pressure of pure propane at about 60 degrees C.  In this climate, that can relatively easily be reached inside a car.  I hope the boot (trunk?) is a little cooler without the glass effect in the passenger compartment.

Very little methane would release the blow off plug on a warm day.  Instead of guessing, I suspect your favoured search engine will allow you to check the typical composition fairly quickly, and possibly even the fill pump pressure for interest.

Those last two pictures are interesting.  I am not familiar with the particular instruments, but putting a magnet on a float inside the vessel would be difficult to connect to like your diagram through the pressure containment wall.  If it has no external connection, it could be placed inside the tank by the manufacturer, but I don't know if this is actually done.  It looks to me like possibly an ultrasonic transducer at the bottom, measuring the depth of liquid in the tank.  I wonder if the second device (labelled calibration) is a similar transducer, placed on the centre line to  indicate when the tank is half full from which point all other volumes can be easily calculated by a little calculus, even allowing for the shape of the ends.

In my work experience I am familiar with ultrasonic probes being used, though usually placed at the top of the tank.  I don't see why they could not be mounted top or bottom, which ever is most convenient, with the appropriate calibration.  Also commonly used are devices utilising a magnet in a float in a stainless steel tube which is detected by a follower on the outside.  This enables measuring levels without jeopardising the pressure containment and causing a leakage source. 

Either way, a microprocessor, such as your Arduino can easily measure the depth quickly enough to allow calculation of an average depth over several measurements to allow for surface sloshing due to the vehicle motion.  This method would give you the most accurate measure with the vehicle stationary, and would not require modifying the tank mounting system.  But of course the ultrasonic device can be expensive.

It would be worth getting more information on this sensor to see whether it is magnetic or ultrasonic, and whether it would give you a proportional output over the depth range.  And also what it would really cost.

I see that Admiral DK has discovered some IC chips that measure magnetic fields.  Definitely worth following up.  You can always get real data sheets from TI which will tell you all you need to know.  Perhaps those instruments are magnetic after all.

It is an interesting problem that needs a solution, you don't want to run out of gas at speed on the freeway.

MJM460
The more I learn, the more I find that I still have to learn!

Offline Pete49

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Re: DIY Measurement fuel quantity and EGT
« Reply #8 on: October 27, 2018, 04:20:32 AM »
I have the duel fuel system in my Nissan Patrol (Fuel injected 4.2l) and it has the system in the last picture and is pretty accurate. I always have a full tank of petrol so I can change over when I run out of gas.
Pete
I used to have a friend.....but the rope broke and he ran away :(

Offline MJM460

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Re: DIY Measurement fuel quantity and EGT
« Reply #9 on: October 27, 2018, 06:38:40 AM »
Hi Pete, do you know how that one works?

Is the sender on the outside of the bottle, or installed through the wall? 

Good to know an accurate system is available.

MJM460
The more I learn, the more I find that I still have to learn!

Offline AlexS

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Re: DIY Measurement fuel quantity and EGT
« Reply #10 on: October 27, 2018, 11:39:35 AM »
Did some test yesterday. It seems that there is a tank gauge with a gear transmission (shown in the image below). It appears that the gears can break if there is an empty tank or something else.
The meter, shown earlier, still appears to work. This meter is a analog sensor and that has a resistance between between 0-95 ohm. The hands and thus the resistance is determined on the basis of the tank gauge. The hands move on the basis of a magnetic connection with the tank gauge.

I let the weight measure as it is, I can use this nicely for my own engine project. Now I see if I can get another tank meter from a specialist. Maybe it is repairable by my self. But first properly degassing the tank and remove the sensor.

But guys thanks for your input!

Offline MJM460

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Re: DIY Measurement fuel quantity and EGT
« Reply #11 on: October 28, 2018, 06:32:26 AM »
Hi Alex, I am glad you solved the problem.  A pity the mechanism broke.  As you say, rattling in an empty tank or inevitable sloshing of the liquid?  Hard to tell.  Perhaps both and just a limited life wearing/tearing component, though not totally easy to replace.

Be careful when degassing.  One thing that may be helpful is that now they use nitrogen in many car tyres, you can probably get nitrogen to pressurise and vent it down a few times.  Much safer than using air, but only outdoors or in a well ventilated space.

I hope the project continues to proceed well.  Did you find out anything more about the gas composition?

MJM460
The more I learn, the more I find that I still have to learn!

Offline AlexS

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Re: DIY Measurement fuel quantity and EGT
« Reply #12 on: November 06, 2018, 07:31:31 PM »
Hey MJM460,

The mechanism seems not broken. Someone has inserted a screw for preventing the float from blocking, but that seems fine. While putting the gauge in the mechanism, the gauge seems follows correct the mechanism by operating the float. And the resistant of the gauge increased by operating the float.

So it looks like it would work. But. Only thing I noticed was that the float might reach the 80% filler limiter (also a kind of float system). Those kind of crosses in the middle of the tank. Maybe someone has changed the filler limiter when turning or bending the float. But my car is equipped with a 60 L tank, and the maximum volume that I have tanked was around 46 liters. So that seems also working correctly. Added a picture with the overview of the fittings and piping of the tank and one of internet source.

So I now did place it back, hoping that maybe the refitting solve the problem. But this does not help, while put some gas in the tank the sensor shows no change. It is very change.

Offline AlexS

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Re: DIY Measurement fuel quantity and EGT
« Reply #13 on: November 06, 2018, 07:52:04 PM »
The second is that the tank should be mounted at an angle of around 52 and 60 degrees.

I went to an LPG specialist later. And he indicated that the angle under which the tank is currently mounted is actually not sufficient. He indicated that you could not fill the tank completely and that maybe the chance that some liquefied gas could be sucked in.
However, it is not possible to mount the tank at a larger angle. You will soon encounter the torsion bars of the trunk.

Offline Admiral_dk

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Re: DIY Measurement fuel quantity and EGT
« Reply #14 on: November 06, 2018, 09:54:00 PM »
I see bad potentiometers almost every day - mind you, they are consumer grade ... but I wouldn't be surprised if it is the culprit here too - even if better quality. Can you change it without disassembling the tank again ?

Best wishes

Per